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SOCAT udp

socat udp


Connected vs unconnected#

UDP sockets can be “connected” (or “established”) or “unconnected”.
- Connected sockets have a full 4-tuple associated {source ip, source port, destination ip, destination port}
- Unconnected sockets have 2-tuple {bind ip, bind port}.

unconnected#

server
socat UDP-RECV:1234 -
client
socat STDIO udp-sendto:127.0.0.1:1234

Connection established only when data send

ss -panu sport = :1234 or dport = :1234 | cat
State    Recv-Q   Send-Q     Local Address:Port     Peer Address:Port  Process  
UNCONN   0        0                0.0.0.0:1234          0.0.0.0:*      users:(("socat",pid=144971,fd=5))

Demo: Test connected server#

server
socat UDP-RECV:1234 -
client 1
socat STDIO udp-sendto:127.0.0.1:1234
# send data
client 2
socat STDIO udp-sendto:127.0.0.1:1234
# send data

connected#

server
socat udp-l:1234 -
client
socat STDIO udp:127.0.0.1:1234

Connection established when call connect

ss -panu sport = :1234 or dport = :1234 | cat
State     Recv-Q    Send-Q       Local Address:Port        Peer Address:Port    Process                                                                         
ESTAB     0         0                127.0.0.1:47917          127.0.0.1:1234     users:(("socat",pid=144636,fd=5))                                              
UNCONN    0         0                  0.0.0.0:1234             0.0.0.0:*        users:(("socat",pid=144633,fd=5))  

Demo: Test connected server#

  • Establish server
  • Client Connect
  • Second client connect and try send data
server
socat udp-l:1234 -
client 1
socat STDIO udp:127.0.0.1:1234
# send data
client 2
socat STDIO udp:127.0.0.1:1234
#
2022/06/28 21:42:56 socat[145414] E read(5, 0x556853768010, 8192): Connection refused

echo server#

terminal1 - server
# udp server
socat -v udp-l:1234,fork exec:'/bin/cat'
terminal1 - client
nc -u 127.0.0.1 1234
# or
socat - udp:127.0.0.1:1234

Broadcasting / Multicasting / Unicasting#

Broadcasting#

server / listener
socat udp-recv:1234 -
client
socat - UDP-DATAGRAM:192.168.1.255:1234,broadcast,sp=11111

# tcpdump
sudo tcpdump -n -i <eth_name> udp
192.168.1.207.11111 > 192.168.1.255.1234: UDP, length 3

summary#

# connected
# server
socat STDIO UDP-LISTEN:11111
# client
socat - UDP:localhost:11111

# unconnected
# server
socat UDP-RECV:11111 STDOUT
# client
socat STDIN UDP-SENDTO:127.0.0.1:11111


# Server
socat UDP-RECVFROM:11111,fork STDOUT

Reference#